1. Field of Invention
This invention relates to an optical frequency analyzer having high accuracy and high resolving power.
2. Description of the Prior Art
A conventional optical frequency spectrum analyzer may be of the following types. (A) One utilizing a diffraction grating or a prism as a spectroscope. (B) One utilizing a Fabry-Perot resonator as a spectroscope.
As illustrated in FIG. 1, two half mirrors HM are disposed to form a resonator. Let the light velocity be c, and let the distance between the half mirrors be L. This resonator has a resonant frequency (see FIG. 2) at a frequency interval of c/2L. When light to be measured is rendered incident upon the left half mirror HM, light having the frequency identical to the resonant frequency is transmitted through the half mirror HM and falls on receiving device PD. When half mirror HM is oscillated by means, for example, of a PZT or the like, in order to sweep the resonant frequency, the spectrum of the measurement light can be observed from the output from light receiving device PD.
In the optical frequency spectrum analyzer described in (A), however, wavelength resolving power becomes 0.1 nm (equivalent to about 30 GHz) or thereabout; and an absolute accuracy is about 2 nm (equivalent to about 600 GHz). These results are not favorable. On the other hand, the optical frequency spectrum analyzer described in (B) shows results of the limit of frequency resolving power to about several tens of MHz. If the measurement if effected by inputting light having a reference wavelength, the absolute wavelength can be measured. The treatment is, however, very difficult, and accuracy is deteriorated (in connection with degreee of parallelism of mirrors and addjustment of perpendicular incidence, or error in frequency caused by fluctuations of an interval at which the mirrors are disposed). Furthermore, there is a defect in that it is impossible to simultaneously measure laser beams which are being oscillated in a plurality of modes.
Frequency measurement with high accuracy of 1 MHz or less and with high resolving power is required in the field of optical communication s and photo applied mesurements. Hence, the above types of optical frequency spectrum analyzers are unsatisfactory.
FIG. 3 is a block diagram depicting a conventional optical fiber loss wavelength characteristic measuring device. Output light from a variable wavelength light source VL enters a fiber MF to be measured, and the subsequent emergent light is detected by a photo detector PD. The detected light is outputted as an electric signal to an amplifying/displaying circuit DP. The characteristics of wavelength are measured from the variations of light power obtained when sweeping the output wavelength of the variable wavelength light source VL.
FIG. 4 is a block diagram showing a conventional optical fiber wavelength dispersion characteristic measuring device. The variable wavelength light source VL and a reference wavelength light source SL are amplitude modulated by a modulation signal source Ef having a frequency f. The photo detector member PD detects output optical powers both of measurement fiber Mf to which the output light of variable wavelength light source VL is applied and of reference fiber SF to which the output light of source SL is applied. The phase differences in component of the frequency f between the two outputs are detected by a phase measuring device PS, thereby measuring a propagation delay time with respect to the wavelength of the measurement fiber MF.
However, the measuring devices depicted in FIGS. 3, 4 are deficient in many respects, such as, the optical phase propagation characteristics cannot be measured in a highly accurate manner. The only acceptable measurement becomes possible with use of a long light path as in the case of optical fibers. A short waveguide path is not acceptable for obtaining accurate measurements. The measurement in regard to the propagation characteristic (e.g. loss, gain, phase, delay) and reflection characteristics is of importance to testing performance of such devices as the optical fiber, light waveguide path, wavelength branching filters, optical switches and the OEIC which are all essential components in any communication system or photo applied measurement systems. The above described conventional devicees and systems are not sufficiently adequate.